What is express.Router and why use it?

A college has one main gate, but each department has its own office handling its own matters. express.Router is the department office. The main app only decides, "Anything starting with /students goes to the students office."

// routes/students.js
const express = require("express");
const router = express.Router();

router.use((req, res, next) => {
  console.log("students route hit:", req.method, req.originalUrl);
  next();
});

router.get("/", (req, res) => res.json([{ id: 1, name: "Anjali" }]));
router.get("/:id", (req, res) => res.json({ id: req.params.id }));
router.post("/", (req, res) => res.status(201).json(req.body));

module.exports = router;

// app.js
const express = require("express");
const students = require("./routes/students");

const app = express();
app.use(express.json());
app.use("/api/students", students);   // paths inside are relative to this prefix
app.listen(3000);

Benefits:

• One file per feature: students, courses, payments.
• Middleware can be applied to a whole group, such as router.use(requireLogin) for admin routes.
• Prefixes can be changed in one line, which makes API versioning easy: app.use("/api/v2/students", studentsV2).

Use express.Router({ mergeParams: true }) if a nested router needs params defined in the parent path.

• Repeating the prefix inside the router, producing /api/students/api/students.
• Forgetting mergeParams when reading the parent's route parameters.